Processing and Characterization of Multi-Layered Wear-Resistant Ceramic Coatings

A new concept of a multi-layered ceramic coating consisting of a porcelain enamel bond interlayer with thermal expansion characteristics tailored to match those of a cast iron substrate and a plasma-sprayed wear-resistant surface layer of chromium oxide, chromium carbide, or partially stabilized zirconia was investigated. Flat substrates of grit-blasted and surface-treated cast iron were slurry sprayed with a finely ground glass frit mixed with isopropyl alcohol and water. Subsequently, the specimens were dried in an oven before firing at 1023 K to produce a smooth, dense, and strongly adherent enamel coating 75 to 100 μm thick. Wear-resistant coatings of oxides and carbides with varying particle sizes and powder densities were then thermally sprayed, by means of a plasma spray gun, at controlled power ratings and gas flow rates. The produced ceramic coatings were subsequently ground to a thickness of about 120 μm. Adherence testing by the tensile pull-off technique revealed high interfacial strength between the enamel coating and the substrate resulting from enhanced chemical interdiffusion and mechanical interlocking. Unlubricated sliding wear experiments using a ball-on-flat tribotester and tungsten carbide balls as sliders demonstrated low initial and moderately high steady-state friction coefficients. Optical and scanning electron microscopy and surface profilometry of the tested ceramic-coated specimens verified that surface damage and wear rate were negligibly small. The important role of the main process parameters on the interfacial adherence and uniformity of the enamel and ceramic layers and the potentiality of the developed processes in the deposition of relatively low-friction and wear-resistant multi-layered thick ceramic coatings are discussed in the context of the obtained results.